Sandwich tern

Thalasseus sandvicensis is a tern of the family Laridae whose scientific name carries the authority Latham, 1787 ^[1]. In Peru it appears as «gaviotín de pata negra» in the bird list of Los Manglares de Tumbes National Sanctuary ^[2]. What is known in detail about its biology comes from studies in Europe and southern Africa, not Peru: the birds that spend the austral summer on the coasts of southern Africa come mostly from colonies on the European Atlantic seaboard and the Baltic, and fewer from the Mediterranean ^[3]. The IUCN Red List classifies it as Least Concern ^[4].

In southern Africa it is a fairly common non-breeding visitor, from Namibia to southern Mozambique, although it concentrates on particular stretches of coast ^[3]. Almost all the birds that arrive there were born in colonies on the European Atlantic seaboard and the Baltic; fewer come from the Mediterranean, and only a few from the Black and Caspian seas ^[3].

It prefers sheltered bays, coastal lagoons and the mouths of large rivers, and few birds reach islands far from the coast ^[3]. At its roosts it gathers in protected bays, salt works, sewage works and river mouths ^[3].

In Europe three large breeding populations are distinguished: the north-west European or Atlantic one, the Black Sea one and the Caspian one ^[5]. In the Mediterranean it breeds mostly at the mouths of large rivers, such as the Rhône, in the Camargue, since 1948, and the Ebro, since 1971 ^[5].

It breeds in colonies, and individuals usually breed for the first time between 3 and 5 years of age ^[5]. Colonies develop unevenly and are sometimes abandoned, in the North Sea as in the Mediterranean ^[5].

It migrates almost always along the coasts of Europe and Africa, and not out to sea ^[3]. Adults reach western South Africa from early September and keep arriving until November, while birds of the year may keep arriving until March ^[3]. Departure from the east coast begins in January, and by mid-March most of the presumed breeders have left ^[3]. According to the European studies the author reports, the journey back is unhurried, much less than 100 km a day, and reaches the breeding grounds from late April into May ^[3].

Some non-breeders of up to 5 years old spend the austral winter on the west coast, where concentrations of up to 250 birds have been seen ^[3]. Within southern Africa movements are continuous: a bird ringed in the Eastern Cape on 11 January was recovered 18 days later, about 850 km north-east, in northern KwaZulu-Natal ^[3].

It is a long-lived species: the oldest bird found in southern Africa had been ringed as a chick in Scotland and turned up almost 27 years and 5 months later, and the longest-lived known was 30 years and 9 months old ^[3]. Exchange between populations is rare: of 4021 European recoveries in the breeding season, only 16 (0.40%) were a move between the Atlantic and the Mediterranean population ^[5].

It feeds mostly close to the coast and by day, within 2 km of the shore or at the mouths of large estuaries ^[3]. Some birds fly out to islands far from the coast to roost with other terns, especially with Thalasseus bergii ^[3].

At the daytime roosts of the Walvis Bay and Mile 4 salt works it perches on the poles in the oyster-rearing ponds ^[3]. Together with Thalasseus bergii, it is among the first to gather at a nocturnal roost, up to an hour and a half before dark, and its arrival serves as a gathering point for other species ^[3].

Numbers vary greatly from year to year at the same site: at Sandwich Harbour, in Namibia, nearly 3800 birds were counted on 24 January 2004, 5400 at the end of January and beginning of February 2005 and only 55 in mid-January 2003 ^[3]. The author supposes these changes follow food availability, although this is not clear ^[3].

The IUCN classifies it globally as Least Concern ^[4]. In the bird table of the master plan of Los Manglares de Tumbes National Sanctuary it carries the same IUCN category and no national category in the column for Supreme Decree 004-2014-MINAGRI ^[2].

For southern Africa a summer population of 10000 to 15000 birds was estimated in a good year and fewer than 10000 in a poor one, under 10% of the western European population, put at 166000 to 171000 birds ^[3].

Among the causes of recovery of ringed birds, 182 records were of birds found dead with no further detail and 13 of birds shot, besides a few shot for museum specimens, apparently for sport ^[3]. In Angola terns were often hunted during Portuguese rule, and young birds were the most exposed ^[3].

The master plan of Los Manglares de Tumbes National Sanctuary gives 158 bird species recorded between 2015 and 2020 ^[2]. The «gaviotín de pata negra» appears in its bird table, which gives no dates or abundances per species ^[2].